Structures such as offshore drilling platforms are located at various locations in the ocean where they may be exposed to high sea states. It is necessary for such structures to be firmly anchored in order to avoid excessive lateral drift relative to a desired location. The anchoring structure used has typically comprised a substantial number of conventional anchors having flukes which dig into the anchoring ground and lengths of chain extending between each anchor and the floating structure. Because of the weight of the chain relative to its strength, this attachment means has limited the depth at which this anchoring technique can be employed. Wire rope, on the other hand, has better characteristics with respect to strength per unit length, but does not in and of itself, form a good suspension, interconnection member between the anchor and the floating structure. However, wire rope, when used alone, tends to impose upward forces on the anchor when unloaded.
It has been found preferable to utilize an anchor line comprising an anchor chain extending from the anchor and a wire rope extending from the opposite end of the anchor chain to the floating structure. The combination of wire rope and chain anchor line allows the maximum weight and holding power of the anchoring system to be concentrated at the end of the anchor line in contact with the anchoring ground or sea bottom with the lighter wire rope carrying the anchoring force between the floating vessel and the anchor chain.
While it is known to utilize a combination anchor chain and wire rope in forming an anchor line, certain drawbacks have been encountered and it is toward the correction of these drawbacks that the present invention is directed.
In the positioning of floating structures by means of combination anchor line systems of the general configuration described above, the anchor line system tends to take on a gradually curving profile or sag from the floating structure to the anchoring ground. As can be appreciated from the prior discussion, differing characteristics in the anchor line are demanded at various points along the entire anchor line. The use of a combination wire rope and anchor chain, for example, addresses the problem of criticality of length to weight ratio encountered in deep water anchoring. Unfortunately, the use of a combination anchor line, while solving one problem, lends itself to additional complications and drawbacks. Due to relatively large movements of the floating structure which take place, the portion of the anchor line proximate the structure is subjected to additional stress and the destructive effects of the harsh salt-air environment augmented by the salt spray as well as the movement of the chain in and out of the salt water. Thus, there is a significant danger of corrosion in the wet-dry zone proximate the surface of the ocean and the floating structure. It is a difficult task to weigh a portion of the anchor system in order to repair a section of the wire rope. Thus, another characteristic which is desirable in the wire rope and anchor chain combination for an anchor line system is strength, corrosive resistively and repairability at the portion of the anchor system proximate the floating structure and ocean surface.
Another desirable characteristic of deep water anchoring systems is a capacity to substantially resist the damaging effects of corrosion at the chain portion proximate the anchoring bed or sea floor.
It is, therefore, an object of this invention to provide an anchoring system which incorporates the features necessary to provide effective, long term deep water anchoring security.
It is another object of this invention to provide an anchoring system in which the section of the anchor system proximate the floating structure is substantially protected from the harsh environment of the salt spray and water.
It is still another object of this invention to provide an anchor system for long term, deep water anchoring which system lends itself to repairs when necessary in the event of corrosive damage to the anchor system proximate the floating structure.
It is yet another object of this invention to provide an anchoring system in which the chain portion of the system proximate the sea bed is substantially resistant to corrosion.